Due to the characteristics of Liquid Crystal Displays (LCDs) such as low power consumption, low voltage operation, lightweight, and thinness, LCDs have recently been used in various display devices. Such LCDs are composed of a variety of materials, including liquid crystal cells, polarizing plates, retardation films, condensing sheets, diffusion sheets, light guide plates, and light reflecting sheets. Therefore, improvements have been actively been made to obtain advancement in productivity, weight reductions, and brightening of the LCDs by reducing the number of films included in the LCDs or decreasing thicknesses of the films or sheets included in the LCDs.
Polarizing plates have been used in structures in which protective films are laminated using adhesives on one side or both sides of polarizers formed of polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resins usually dyed with dichroic dyes or iodine. Although triacetyl cellulose (TAC)-based films as polarizing plate protective films have been mainly used, there have been problems that the TAC films were easily deformed at high temperature and high humidity conditions. Therefore, protective films formed of various materials that are capable of replacing the TAC films have recently been developed. For example, methods of using polyethylene terephthalate (PET), cycloolefin polymer (COP), acrylic films, or mixtures thereof as the protective films have been suggested.
Studies have been actively made into a technology of using polyethylene terephthalate (PET) as the protective films among the above-detailed methods, since polyethylene terephthalate (PET) films are inexpensive and have excellent transparency.
A water-based adhesive layer formed between a polarizer and a polyethylene terephthalate (PET) film, and polarizing plates between which the adhesive layer is interposed are disclosed in Korean Patent Publication Nos. 2011-0075998, 2010-0068178, etc. However, since water-based adhesives are used in previous technologies, there is a problem that durability is deteriorated due to insufficient adhesive strength between the polarizer and the polyethylene terephthalate (PET) film. Further, it is general that polyethylene terephthalate is used on one side of the polarizing plate, and other types of polymer films having excellent optical properties or characteristics such as permeability and so on are used on the other side of the polarizing plate when manufacturing a polarizing plate since the polyethylene terephthalate (PET) film has low permeability or optical properties even though being inexpensive. However, when a water-based adhesive is used as an adhesive in a polarizing plate using different types of polymer films on both sides of the polarizer, there are problems that a dry efficiency difference is generated by a difference in water permeability between a polyethylene terephthalate film and a polymer film that is different from the polyethylene terephthalate film in the drying process for hardening the adhesive, and curling of the polarizing plate is intensified consequently. If curling is generated in the polarizing plate, there may be problems that it is difficult to adhere to the polarizing plate in a lamination process of adhering the polarizing plate to an image display device, and a defect ratio may be increased due to the inflow of foreign objects. Further, even after adhering the polarizing plate to the image display device, a defective contact with a liquid crystal panel or a bending phenomenon of the liquid crystal panel is easily generated by curling of the polarizing plate. Accordingly, since a large amount of strain may be formed while a contact of the display device module with a module case is occurring when the image display device is mounted on a display device module, light leakage or unevenness may be generated, causing image defects.
Therefore, it is necessary to develop polarizing plates which are excellent in terms of optical properties and curling characteristics while using polyethylene terephthalate films as protective films.